We have developed a new method for investigating quantitative aspects of medical decision-making. Thresholds are the probabilities of disease at which a clinician will decide to use a diagnostic test or to initiate therapy. The threshold concept has previously been advanced as a means of calculating theoretically optimal probabilities to guide decision-making. These threshold probabilities can be obtained using decision analytic techniques. In contrast, we describe a method of deriving a physician's (or group of physicians') actual thresholds governing clinical decisions, a descriptive rather than normative approach. Our approach uses both hypothetical patient management problems and actual clinical decisions. For this project, we have developed protocols to apply the technique of threshold analysis to the management of patients with coronary artery disease. We will examine the decision to use exercise thallium testing and coronary arteriography in both pencil-and-paper exercises and in an actual clinical setting. The aims of this project are to: 1) Measure physicians' thresholds by the newly developed method; 2) Compare this derived threshold with a decision analytic threshold and a self-reported threshold; 3) Contrast derived thresholds among groups of physicians; 4) Contrast derived thresholds for patients of different ages and socioeconomic groups; 5) Utilize derived thresholds to calculate physicians' estimates of benefit-risk ratios; 6) Assess the effects of a decision aid on derived thresholds; and 7) Contrast quantitative assessment of derived thresholds in hypothetical patient scenarios and in actual clinical decisions.